1. Field of the Invention
The present invention relates broadly to steerable antennas having phased arrays of quadrifilar or other multifilar antenna elements. More particularly, the present invention concerns an antenna comprising a phased array of quadrifilar or other multifilar antenna elements (e.g., quadrifilar helix elements) fed power signals by a time-delaying mechanism (e.g., coaxial cables of differing lengths, or a signal processing unit comprising a microcontroller or microprocessor), wherein the time-delaying mechanism functions to cause the propagation time of the power signals to change and thereby shifts or steers the antenna""s main beam.
2. Description of the Prior Art
It is often desirable, particularly in, for example, satellite, radar, and various military applications, to have and use a low cost lightweight microwave communication antenna capable of maximum range coverage and fast tracking times. Unfortunately, prior art antennas are extremely heavy and cumbersome and do not provide the desired degree of performance.
Phased array antennas are well-known and comprise several antenna elements grouped together, typically spaced apart from one another in an organized positional arrangement which can be linear, two-dimensional, or three-dimensional. In a two-dimensional array, the elements can be positioned, for example, on a rectangularly spaced grid, a triangularly spaced grid, or a circularly spaced grid. This organized arrangement results in a desirable radiation beam pattern, typically consisting of a single main beam or main lobe, which is dependent on the amplitude and phase, either transmitted or received, of the power signal of each element. It will be appreciated, however, that the array needs not be organized in any particular pattern. Thus, the array can be arranged in a so-called xe2x80x9cthinned arrayxe2x80x9d, wherein some of the elements are not needed and are eliminated, thereby resulting in a somewhat disorganized array configuration. Phased array antennas are preferred over standard single antennas because they provide higher gain capabilities with a narrower main beamwidth and increased tracking speeds using rapid electronical beam steering.
Quadrifilar helix antennas (QHAs) are also well-known and comprise four equal length electrical conductors or arms spaced 90xc2x0 apart and wrapped in helical form around a fixed radius. In recent years, the QHA has developed into an excellent radiator for use in mobile and weather satellite applications. The QHA is similar to other antennas in that the crossed dipole and the patch radiate similarly shaped patterns. The QHA is preferred, however, for a number of reasons, including, for example, that it requires no ground plane, which allows for substantially reducing its weight and size and makes it ideal for low-weight applications, such as, for example, on-board satellite communication applications. Furthermore, the QHA""s radiation pattern can be controlled by varying structural parameters, which is ideal for design purposes. Additionally, the QHA""s elevation radiation pattern has a wide circular polarized beam, making it an excellent radiator for satellite communications tracking systems.
In the prior art, QHAs are used for communications applications, but only in single unit, compact installations. Phased array antennas incorporating multiple QHA elements have not been used because it is thought that the omnial characteristics of the QHA would result in inefficiency in a phased array configuration. Furthermore, phase-shifting involved in the operation of a QHA introduces additional complications into the design of a phased array antenna. Thus, directional antennas, such as, for example, horn or reflector-type antennas, are deemed more feasible for use in a phased array antenna.
Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved phased array antenna.
The present invention overcomes the above-described and other problems and disadvantages in the prior art by providing an antenna comprising a phased array of quadrifilar or other multifilar antenna elements fed power signals by a time-delaying mechanism, wherein the time-delaying mechanism functions to cause the propagation time of the power signals to change and thereby shifts or steers the antenna""s main beam. As described herein, the antenna elements are QHA elements, and the time-delaying mechanism includes replaceable coaxial cables of differing lengths adapted to feed each element with a separate time-delayed power signal, either transmitted or received. Alternative and equally suitable time-delaying mechanisms may be implemented using software, firmware, hardware, or any combination thereof, and may be based upon such devices as logic gates, microcontrollers, or microprocessors, rather than replaceable coaxial cables. By changing the time delay or shifting the phase angle of the elements"" feed signals, the individual radiation patterns formed by the elements combine and interact with one another to produce the main beam which is shifted at a desired angle.
Each element includes four identical helical-shaped electrically conducting arms spaced 90xc2x0 apart at a fixed radius. Different radiation patterns can be achieved by feeding the elements with different phase-shifting configurations. A cardiod-shaped radiation pattern, for example, can be achieved if each element is provided with a power signal having an identical amplitude but a 90xc2x0 shift in phase relative to each other element.
Each element is constructed on a small PCB which is tightly-rolled and encased in a polymer-type cylindrical housing. The arms of the element are constructed on the PCB as four etched copper traces which, when the PCB is rolled into cylindrical form, produce the helical shape of the QHA. Each element is provided with a built-in quadrature phase-shifting feed network implemented as a microstrip phase-shifting circuitry etched on the inside of the PCB cylinder, which advantageously makes additional phase-shifting components unnecessary. This built-in phase-shifting feature also allows for an overall reduction in the number of required interconnects.
The elements are mounted on a main PCB which is constructed from the RF dielectric material. The PCB has a first side used as both a ground plane and as a mounting foundation for the sixteen elements, and a second side used to provide a microstrip feed circuitry for feeding the elements. The microstrip feed circuitry use an equal line-length planar feed geometry etched to a +/xe2x88x925 mil width tolerance and adapted to provide an equal power feed delay to the individual elements. Five three-branch Wilkinson power dividers are used to further ensure that equal power is delivered to each element.
The time-delaying mechanism includes four time-delay feed lines, with each feed line having four coaxial cables, such that each element is fed by a respective one of the coaxial cables. The coaxial cables bridge the microstrip feeding circuitry to the elements and provide, respectively, a 0, t, 2t, and 3t time delay. By swapping coaxial cables of different lengths, various time delays can be configured and the main beam can be steered. More specifically, uniformly altering the physical lengths of the coaxial cables results in corresponding alterations in the time delay to the elements. By changing the lengths of the coaxial cables, the propagation of the power signal is either increased or decreased, causing the individual elements to take on various beam shift angles and thereby resulting in a shift in the antenna""s main beam.
Thus, it will be appreciated that the antenna of the present invention provides a number of substantial advantages over the prior art, including, for example, that, by incorporating QHA elements, it advantageously provides a higher gain and a narrower beamwidth. Furthermore, the antenna advantageously provides better circular polarization characteristics, and lower sensitivity to a ground plane, which allows for weight and cost-saving elimination of the ground plane. Additionally, the antenna can be manufactured at a substantially lower cost than most other antennas with similar radiating characteristics. Additionally, the main beam can scan electronically at very high speeds by varying the time delay. For these reasons, use of the antenna in the commercial, military, industrial, scientific, and medical (ISM) band provides excellent circular polarization in a steerable format. Applications for the antenna include commercial satellite, mobile base station, and indoor wireless communication applications.
These and other important features of the present invention are more fully described in the section titled DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT, below.